Highside FET extending on time indefinitely

H

Hammy

Guest
This is from AN978 pg18 using IR's bootstrapped HS drivers.

http://i40.tinypic.com/1gljdv.png

You can see from the picture they have the timer tied to ground
through a 100k resistor why not just use the HS driver's 12Vdc return?
Is there any reason this wouldn't work with the 555 ground tied to the
same return as the IR2125.

I'm trying to do something similar keep a HS Fet on for extended
periods. I'm using a FAN7361 HS driver. I have a PIC operating off the
same 16VDc supply that the driver is operating off through a 5V shunt.
Could I use one of the PIC's outputs to recharge the boost cap in the
FAN7361 driver so it can keep the FET on until the PIC tells it to
shutdown?

FAN7361 data sheet

http://www.fairchildsemi.com/ds/FA%2FFAN7361.pdf

AN978 (pg18)

http://www.irf.com/technical-info/appnotes/an-978.pdf
 
On Feb 14, 8:54 pm, Hammy <spa...@hotmail.com> wrote:
This is from AN978 pg18 using IR's bootstrapped HS drivers.

http://i40.tinypic.com/1gljdv.png

You can see from the picture they have the timer tied to ground
through a 100k resistor why not just use the HS driver's 12Vdc return?
Is there any reason this wouldn't work with the 555 ground tied to the
same return as the IR2125.

I'm trying to do something similar keep a HS Fet on for extended
periods. I'm using a FAN7361 HS driver. I have a PIC operating off the
same 16VDc supply that the driver is operating off through a 5V shunt.
Could I use one of the PIC's outputs to recharge the boost cap in the
FAN7361 driver so it can keep the FET on until the PIC tells it to
shutdown?

FAN7361 data sheet

http://www.fairchildsemi.com/ds/FA%2FFAN7361.pdf

AN978 (pg18)

http://www.irf.com/technical-info/appnotes/an-978.pdf
Click to expand...
Hi Hammy,

as far as i'm concerned, they are tied to the same ground.
the reason there's a 100k resistor is because they need to limit the
current trough that zener diode.
the zener diode is used to clamp the input voltage for the timer power
supply to 14.4V. but you need to limit the break down current on that
diode other wise it will blow. the 100k resistor is doing that.

for the second point, no, you need higher voltage then the source to
keep the FET on. so i would suggest a charge pump IC to do it.

what type of application are you trying to achieve? what's the voltage
on the rail?
maybe you are using the wrong part. this is the high speed switching
driver, but not the ON and OFF control switch.
 
On Sun, 15 Feb 2009 10:23:53 -0800 (PST), tomrei
<tomrei2000@gmail.com> wrote:


maybe you are using the wrong part. this is the high speed switching
driver, but not the ON and OFF control switch.
Click to expand...
Yes maybe I'll look at mechanical relays again.
 
On Feb 15, 7:49 pm, Hammy <spa...@hotmail.com> wrote:
On Sun, 15 Feb 2009 10:23:53 -0800 (PST), tomrei

tomrei2...@gmail.com> wrote:
maybe you are using the wrong part. this is the high speed switching
driver, but not the ON and OFF control switch.

Yes maybe I'll look at mechanical relays again.
Click to expand...
well, i know some part maybe can help,
that's why i asked what's the voltage on the rail you plan to
control...

the ADM4210 can act as a switch,
it's a hotswap controller used to protect the circuit by current
limiting the inrush current when you hot plug power.
it has build in charge pumps used to control the external FET in turn
control the rail.
if you want to limit the current and do some protection, use the sense
resistor.
if you don't need that and just want a logic controlled switch, then
just short the Vcc and sense pin, tied the timer pin to high and use
the ON pin to control the on and off of the FET.
beware this part can only operate to maximum of 16.5V rails. for
higher rail, you may need to control the low side instead.

hope it helps

yours Ren
 
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